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Ask Joe Powder
"Ask Joe Powder" is a question and answer column authored by Kevin Biller of
the Powder Coating Research Group. Mr. Biller has over 30 years experience
formulating and manufacturing powder coatings. He welcomes your questions
regarding powder coating technology. Please write to: firstname.lastname@example.org
The Powder Coating Research Group
15 W. Cherry Street, 3rd Floor
Columbus, OH 43215
I’m having a big problem with my
powder coating chipping off my customer’s parts. These are automotive seat
assembly brackets. First we media blast the parts with aluminum oxide to
remove any rust, then wipe clean with acetone. Next we apply 2.0 to 3.0
mils of a zinc-based epoxy powder primer, curing per our supplier’s
recommended bake conditions (20 minutes at 355°F) then topcoating with a
standard bake polyester at about 3.0 mils. The polyester is cured per the
20 minutes at 375°F bake recommended on the technical data sheet provided
by the powder manufacturer. The loss of adhesion occurs between the epoxy
primer and the polyester topcoat. Could we be applying the powder too
thick? Is the powder undercured? Should we bake the powder longer or at a
Your advice will be greatly appreciated.
Jamie Wells – RPM Powder Coatings, Columbus, Ohio
Thank you for the question. By applying a two coat system you really are
giving your customer a “Cadillac” coating that should beyond the life of
the vehicle. When marrying two of more coatings on a substrate there are
certain procedures that can be used to ensure excellent adhesion and
Let’s go through your process. Your metal preparation appears to be sound.
Media blasting can provide an excellent surface for an epoxy primer. It is
important to keep your media clean and to thoroughly blast the entire
surface. A subsequent acetone wipe is a good idea also as long as you use
clean, lint- free wipers.
You mentioned that you are using a zinc-rich epoxy primer. This technology
can provide outstanding corrosion protection of ferrous substrates. It is
important to note that epoxies are incredibly durable and quite hard when
completely cured. Because of this it is wise to under-bake an epoxy based
powder if it is being used as a primer. By under-curing the epoxy primer
you are leaving a softer surface for the polyester topcoat to “bite” into.
Furthermore under-curing provides a few unreacted functional
groups that can be chemically bound by the polyester. These unreacted
groups are compatible with the carboxyl functional groups in the polyester
and provide a much more intimate bond between the primer and topcoat.
I would recommend under-curing the primer by about 25°F (about 15°C) or
for 50% of the recommended time at temperature. I would then apply the
polyester topcoat and ensure that it receives the full recommended bake.
It is also important to make sure the topcoat cures at a higher
temperature than the primer. Otherwise you risk the possibility of
undercuring the entire coating system.
Good luck with your project.
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